(1) Field of the Invention
This invention relates to an improved dyestuff-containing microscopic capsule suspension for record materials, which capsules are prevented from coloration, and more particularly to a suspension in a liquid medium of microscopic capsules of a hydrophobic solvent solution containing an electron donative dyestuff which capsules are prevented from coloration and adopted to produce record materials such as pressure sensitive recording paper.
(2) Description of the Prior Art
As a recording system making use of the color reaction through the mutual contact between a wide variety of electron donative dyestuffs and electron acceptive acidic developers, there have been known pressure sensitive recording paper and the like.
The production of such pressure sensitive recording paper has been considerably increased in recent years as carbonless duplicating paper (i.e., non-carbon paper) with the trend of office work rationalization and the popularization of computers. Its demand is expected to increase still further in the future.
Pressure sensitive recording paper was first rendered marketable upon completion of the microencapsulation technology for a solution containing an electron donative dyestuff, taking the hint from the color reaction between crystal violet lactone (hereinafter, abbreviated as "CVL") and acidic terra abla. Owing to the subsequent technology improvement in various fields such as dyestuffs, developers, solvents for dyestuffs, microencapsulation technique and coating technique, the quality and performance of pressure sensitive recording paper have been steadily improved.
As electron acceptive acidic developers, in addition to acidic terra abla which has been used from the dawn of pressure sensitive recording paper, other developers have been proposed and actually used, including phenol-formaldehyde polymer, metal-modified phenol-formaldehyde polymer, substituted salicylic acids and their multivalent metal salts.
As electron donative dyestuffs, a number of dyestuffs have been proposed including (1) various phthalide dyestuffs led by CVL; (2) various fluoran dyestuffs; (3) various azaphthalide dyestuffs; (4) leucoauramine dyestuffs; (5) phthalan dyestuffs; (6) spiropyran dyestuffs; (7) acylleucophenothiazine dyestuffs; (8) diphenylmethane dyestuffs; and (9) triphenylmethane dyestuffs. In accordance with the development of new developers, besides CVL (phthalide) and benzoylleucomethylene blue (acylleucophenothiazine) that have actually been used from the beginning, varied types of phthalide dyestuffs, fluoran dyestuffs and azaphthalide dyestuffs have been adopted for actual use or are about to be used actually.
These dyestuffs are dissolved in a dyestuff solvent and encapsulated for use in the production of pressure sensitive recording paper. In such microcapsules, in place of polychlorinated biphenyls which were employed in the beginning, other hydrophobic solvents of low toxicity and high boiling point have been proposed and actually used including partially hydrogenated terphenyls, alkyldiphenyls, alkylbenzenes, alkylnaphthalenes, diallylalkanes and alkyldiphenylethers.
Regarding the microencapsulation method of the dyestuff-containing solvent, in addition to the microencapsulation making use of the gelatin-type coacervation method which was employed in the initial stage of the microencapsulation technology, a wide variety of microencapsulation techniques which are improved in both quality and applicability and make use of synthetic resin (for example, urea-formaldehyde, melamine-formaldehyde, polyamide and polyurethane resins, etc.) have been proposed. Some of such new microencapsulation techniques have already been employed in actual production.
Owing to the above-described development of varied relevant techniques, it has been feasible with presently available pressure sensitive recording paper to form color images of varied hues such as red, green, black, purple and yellow deep and stably on a surface coated with a developer, although conventional pressure sensitive recording paper could develop blue color only.
However, many of phthalide, fluoran and azaphthalide dyestuffs, which are used extensively as dyestuffs for pressure sensitive recording paper, are liable to coloration during their microencapsulation steps through their dissolution in a hydrophobic solvent of a high boiling point and subsequent microencapsulation of the thus-formed dyestuff-containing solvent in accordance with varied methods or during the storage of the thus-prepared microcapsule suspension. Furthermore, certain pressure sensitive recording papers (CB-paper) are colored on the surfaces coated with such dyestuff-containing microcapsule suspension or are gradually colored during their storage. This coloring problem has been considered to be a serious problem in the production technology of pressure sensitive duplicating paper and a solution thereto has been earnestly waited for.